1. Field of the Invention
The present invention relates to mainframe native tape attachment storage systems built from legacy designs in which the host functions with storage subsystems (Control Units (CUs)) that are each attached to a maximum number of tape drives of the same model type, and more specifically, to the configuration of a single physical CU to support multiple logical control units (LCUs) for different tape drive model types in which each LCU is attached to a maximum number of tape drives of the same model type.
2. Description of the Related Art
In a mainframe native tape attachment storage system, a host communicates directly with a physical tape drive via a physical control unit (CU). The host and physical CU are different types of computer servers. The host runs an operating system such as z/OS or z/Linux. The host is connected through a storage area network (SAN) to one or more physical CUs. Each physical CU is attached via a customer provided SAN, embedded switches or cables to a number of tape drives. The host operating systems have been built from legacy designs to function with CUs containing up to 16 tape drives of the same model type. The CU cannot accommodate greater than 16 tape drives or drives of different model types (e.g. different generations). If the customer has more than 16 tape drives of a given model type or tape drives of different model types, the customer has to purchase and configure multiple physical CUs.
The host defines a group of devices of the same type as a CU. The definition contains the address of the CU. Each tape drive is addressed with a device id (e.g. an offset from the CU's address). For example, a CU may have an address 1B0X where the address is in hexadecimal and the “X” is the offset of the tape drive to be addressed. The tape drives may have addresses 1B00, 1B01, 1B02 and so forth. The host addresses the physical CU and the device id tells the CU which drive is being addressed. This is done by using an inter system communication protocol such as Fibre Connection (FICON) or Small Computer System Interface Logical Unit Number (SCSI LUN) that supports addressing end points by group and then by device. When the host addresses the CU, the CU checks the device id and communication is sent to the appropriate physical tape drive. The CU translates between the FICON traffic that the host understands and the Fiber Channel Protocol (FCP) traffic that the drive understands. The CU is attached to a maximum of 16 drives via a FCP interface. Between the CU and the tape drives is an FCP fabric. There are several forms this can take; in the simplest form this could be nothing more then a cable between the CU and a single drive. It could also be a set of redundant FCP switches or some larger FCP storage area network.
To add a new tape drive to an existing CU, assuming the CU has not reached capacity and the new tape drive is the same model type as the group of drives, the tape drive is physically connected to the physical CU via the FCP fabric. The CU is instructed through service menus to discover the newly attached tape drive. Once configured on the CU, the new tape drive is available for the host to bring online and start using.
To accommodate different model types of tape drives or a number of tape drives in excess of 16, additional CUs must be brought online. The hardware for another physical CU needs to be installed, cabled and powered. A physical CU resides in a frame in a customer's data center. The frame is about 6 feet tall and weighs several hundred pounds. A typical frame will hold 4 physical CUs. Once the hardware is installed, service personnel install microcode on the CU and verify the CU is able to see the attached tape drives. The host defines the group of attached devices as a CU including the addresses of the CU and the device ids for each attached tape drive.